Powder metallurgy, i.e., pressing powdered metals in die presses to make parts, is used by a variety of industries as an inexpensive source of parts. Steel powders are used in the presses to produce structural parts. In general, raw material steel powders are press molded to produce a green compact, the green compact is sintered and heat treated, and then further subjected to coining, forging heat-treatment, cutting, etc. to produce a final product having relatively high strength. Structural steel parts that are fabricated using presently available powder metallurgy techniques and materials are however, generally not very dense, have fair to moderate fracture toughness and have relatively low green strengths thus requiring prolonged sintering at very high temperatures. Typical green strengths for commercial high strength steels range from 1.5 to 4 Kpsi but are for the most part lower than 2.0. Further, the low green strength makes it very difficult to "green machine" the parts as they are removed from the press and following sinter hardening, they are nearly impossible to machine.
Complex part shapes such as class 9 and 10 helical gears, other high precision gears, and sprockets with tight dimensional tolerances cannot, in general, be made from these high strength steels by powder metallurgy using present techniques because the required high temperature sintering step (to increase the density of the part) distorts the part from its original shape and thus requires secondary operations such as grinding, turning or surface densification. Such complex parts are therefore individually machined using expensive techniques. Thus, there exists a need for materials from which high green strength parts can be made to avoid prolonged high temperature sintering and minimize distorting the shape of the pressed parts. There also exists a need for parts that can be green machined.
In order to fabricate high strength steel powders the methods commonly known as premixing and prealloying are used. Premixing is a method of mixing an iron powder with a metal or metalloid powder or an alloy powder, compacting them and subsequently sintering the compact under heat to solid-solubilized these added metals and in some cases added carbon or phosphorus. This method is less than ideal because the added metal powder in the iron powder causes separation or segregation due to the difference between the respective specific gravities and particle shapes of the iron powder and the additive powder(s). This then leads to a problem of part quality by causing wide variability in the strength and the size of the sintered product. This problem is especially pronounced in very small parts.
Prealloying involves using an alloyed steel powder in which alloying elements such as nickel, carbon, copper, molybdenum and chromium are solid-solubilized into the iron before compaction. This method is used to avoid the separation problems of premixing. U.S. Pat. No. 5,240,742 to Johnson et al. provides a variation of such a prealloying method. These partially alloyed powders are then compacted and the compacts are subjected to high temperature sintering. This process does however, have its disadvantages. Namely, since the alloyed steel powder obtained by such prealloying processes is relatively hard when compared with pure iron powder, compaction density cannot be increased sufficiently during compaction making it difficult to obtain a green product of high density, hence the subsequent requirement of high temperature sintering. Accordingly, in prealloying processes such as that of Johnson et al., full advantage of the superior physical properties of alloyed steel cannot be taken. Additionally, the chemical precursors of Johnson et al. have the potential to introduce contaminants into the finished pressed part. High temperature sintering also makes the powders and method of Johnson et al. particularly unsuitable for making parts of complex geometry and tight dimensional control. U.S. Pat. No. 5,628,046 to Dautzenberg et al. teaches a process for fabricating sintered articles from a molybdenum containing steel alloy. These articles are said to have increased as pressed density. These as-pressed articles are however not very green strong and cannot therefore be green machined.
Thus there further exists a need for a method by which the green strength and other properties of pressed steel structural parts can be increased which overcomes the disadvantages of prior art materials and methods. Additionally, as part sizes become smaller and smaller, any segregation in the composition of the parts becomes magnified. Thus, there also exists a need for high strength steel parts that are extremely uniform in overall composition.